Subject: Re: Header Stripping in Cellular

From: Mikael Degermark <[email-masked]>
Date: Fri, 22 Oct 1999 20:39:19 +0200

Zoran, 

At the coming IETF in Washington DC, there will be a BOF discussing
robust header compression for cellular. One of the proposals to be 
presented at that BOF can compress RTP/UDP/IP down to 2 bytes (or
possibly less in coming releases!) even in the presense of substantial
loss on the link. 

Please come to the BOF. Even if we rehash this here, we will have to come
to some agreement as to what the IETF should do about header stripping 
and similar ideas. The BOF is a good venue to hash this out!

Cheers, 

Mikael Degermark (who will chair the BOF)

(I've commented your letter at some places below.)

At 12:37 PM 99/10/22 -0400, Zoran Kostic wrote:
>Header Stripping in Cellular
>
>This topic was touched on briefly before, and the consensus was
>Do not allow it to be put into any standard !!
>
>Let me be the devil's advocate, which does not mean that the principal
>idea appeals to me any more than to anyone else on this list.
>
>I am afraid that  header stripping will almost inevitably be included
>at least as an option in standards that deal with cellular. And service
>providers will probably choose that option as a default operation mode.
>
>This is why:
>
>* Cellular service providers have paid a LOT for bandwidth
>* Improvements over current 2nd generation cellular systems consist
>   to a large degree of improving spectral efficiency of cellular
>   networks (bits/second/hertz/cell) - improving the capacity -
>   improving the revenue flow

With suitable header compression it is possible to get within less than a
dB from circuit-switched voice. So the incentives aren't quite so large
as you say. 

>* Pretty much every single bit is accounted for in framing of RLPs,
>   speech blocks etc...
>* If there is even a remote chance that some bit does not have to be
>carried
>   over the air, it should not be carried over the air, because spectral
>
>   efficienty suffers, cellular capacity gets hurt, revenue and profit
>go down
>
>The questions we have to ask us are:
>
>1) What might be the highest compression rate achievable for TCP/IP
>headers ?
>    Even it is 10, I believe that stripping will be what service
>providers
>    would opt for. And can we even come close to 10 ?

Do you mean TCP/IP or RTP/UDP/IP?
For TCP/IP I find it difficult to go below 4 bytes. 
Which is 10.

For RTP/UDP/IP 2 bytes or less seems within reach (provided the UDP
checksumming issues can be resolved in a reasonable manner.)
Which would be 20. 

>2) If we buy into the argument that header stripping will be legitimized
>
>    regardless of our opinion, what should IETF do about it ? Maybe
>there
>    should be something that from spectral efficiency mimicks header
>    stripping but is more acceptable to this community (again I am not
>    sure that compression will do the job).

Please come to the BOF!

>3) Heretic thought: Cellular services are a kinda special, since the
>base does many things
>    for its mobiles anyway... do we have to be exclusive on the topic of
>header
>    stripping and recreation ? Maybe we should come up
>    with recommnedations  on how should header stripping and recreation
>    be done ??

Perhaps if one could somehow view the cellular link as a kind of RTP mixer
it could be legal to fiddle with the IP/UDP headers. The RTP header must,
as far as I can see, be recreated *exactly*. 

One thing the cellular people seem to like is "All-IP" or "IP all-the-way"
which 
implies that the data is carried using IP end-to-end. If header stripping is 
used, that seems to be a mis-nomer. (Not that that would stop any marketing
people from using it...)

Personally, I believe it is possible to design a sufficiently good header
compression 
scheme that will satisfy both die-hard IP people and the spectrum-efficiency
hungry cellular people! 

Micke